Semiconductor and process for fabricating the same

ABSTRACT

A process for fabricating a semiconductor by crystallizing a silicon film in a substantially amorphous state by annealing it at a temperature not higher than the crystallization temperature of amorphous silicon, and it comprises forming selectively, on the surface or under an amorphous silicon film, a coating, particles, clusters, and the like containing nickel, iron, cobalt, platinum or palladium either as a pure metal or a compound thereof such as a silicide, a salt, and the like, shaped into island-like portions, linear portions, stripes, or dots; and then annealing the resulting structure at a temperature lower than the crystallization temperature of an amorphous silicon by 20 to 150° C.

This application is a Divisional of application Ser. No. 08/636,819,filed Apr. 23, 1996 U.S. Pat. No. 5,879,977, which is itself is aContinuation of application Ser. No. 08/195,714, filed Feb. 14, 1994,now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a crystalline semiconductor for use inthin film devices such as thin-film insulated-gate field-effecttransistors (hereinafter referred to simply as "thin film transistors"or "TFTs"), and to a process for fabricating the same.

2. Prior Art

Thin films of crystalline silicon semiconductor for use in thin filmdevices such as TFTs known heretofore have been fabricated bycrystallizing an amorphous silicon film formed through plasma CVD(chemical vapor deposition) or thermal CVD, using an apparatus such asan electric furnace maintained at a temperature of not lower than 600°C. for a duration of 12 hours or longer. Thin films of crystallinesilicon semiconductor having sufficiently high quality (for example, anexcellent field effect mobility and a high reliability) are availableonly after subjecting the amorphous film to a heat treatment for a stilllonger duration.

However, those prior art processes for obtaining thin films ofcrystalline silicon semiconductor suffer various problems yet to besolved. One of the problems is the low throughput which increases theprocess cost. For instance, if a duration of 24 hours is required forthe crystallization step, and by considering that the process time for asingle substrate is preferably within 2 minutes, 720 substrates must beprocessed at a time. However, the maximum number of substrates which canbe treated at a time in an ordinary tubular furnace is limited to 50; ina practical treatment using only one apparatus (reaction tube), it hasbeen found that a single substrate requires 30 minutes to complete thetreatment. In other words, at least 15 reaction tubes are necessary tocomplete the reaction per single substrate in 2 minutes. This signifiesthat such a process increases the investment cost and thereforeincreases the product price due to a too large depreciation for theinvestment.

The temperature of the heat treatment is another problem to beconsidered. In general, a TFT is fabricated using a quartz glasssubstrate comprising pure silicon oxide or an alkali-free borosilicateglass substrate such as the #7059 glass substrate manufactured byCorning Incorporated (hereinafter referred to simply as "Corning #7059substrate"). The former substrate has such an excellent heat resistancethat it can be treated in the same manner as in a conventional waferprocess for semiconductor integrated circuits. However, it is expensive,and, moreover, the price increases exponentially with increasing area ofthe substrate. Thus, at present, the use of quartz glass substrates islimited to TFT integrated circuits having a relatively small area.

On the other hand, alkali-free borosilicate glass substrates areinexpensive as compared to those made of quartz glass, however, theyhave shortcomings with respect to their heat resistance. Since analkali-free glass substrate undergoes deformation at a temperature inthe range of from 550 to 650° C., and more particularly, since a readilyavailable material undergoes deformation at a temperature as low as 600°C. or even lower, any heat treatment at 600° C. causes an irreversibleshrinkage and warping to form on the substrate. These deformationsappear particularly distinctly on a substrate having a diagonal lengthof more than 10 inches. Accordingly, it is believed requisite to performthe heat treatment on a silicon semiconductor film at a temperature of550° C. or lower and for a duration of within 4 hours to reduce theentire process cost.

SUMMARY OF THE INVENTION

In the light of the circumstances as described in the foregoing, anobject of the present invention is to provide a semiconductor in whichthe problems above are overcome and a process for fabricating the same.Another object of the present invention is to provide a process forfabricating a semiconductor device using the same semiconductor.

The present invention provides a process which is characterized in thatit comprises: forming, on an amorphous silicon film or on a film whichhas such a disordered crystalline state that it can be regarded as beingamorphous (for example, a state which comprises crystalline portions andamorphous portions in a mixed state), a film, particles, clusters,lines, and the like containing at least one of nickel, iron, cobalt,platinum and palladium; and annealing the resulting structure at atemperature lower than the crystallization temperature of a conventionalamorphous silicon by, preferably, 20 to 150° C., or at a temperature nothigher than the glass transition temperature of the glass materialconventionally used as a substrate, e.g., at 580° C. or lower.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) to 1(C) show schematically drawn step-sequential structuresobtained in a process according to an embodiment of the presentinvention (Example 1), as viewed from the upper side;

FIGS. 2(A-1), 2(A-2), 2(B), 2(C) and 2(D) show schematically drawnstep-sequential cross section structures obtained in another processaccording to another embodiment of the present invention (steps ofselective crystallization);

FIG. 3 shows the relation between the crystallization rate of siliconand the temperature of crystallization;

FIG. 4 shows the result of Raman scattering spectroscopy of acrystalline silicon film obtained in an Example;

FIG. 5 shows the X-ray diffraction pattern of a crystalline silicon filmobtained in an Example;

FIGS. 6(A-1), 6(A-2), 6(B), 6(C) and 6(D) show schematically drawnstep-sequential cross section structures obtained in a process forfabricating a semiconductor according to a yet another embodiment of thepresent invention;

FIGS. 7(A) to 7(D) show schematically drawn step-sequential crosssection structures obtained in a process for fabricating a semiconductoraccording to a further another embodiment of the present invention; and

FIG. 8 shows a nickel concentration distribution in a crystallinesilicon film.

DETAILED DESCRIPTION OF THE INVENTION

Conventionally proposed methods for the crystallization of a siliconfilm include a solid phase epitaxial growth from a seed crystal, usingcrystalline island like films as the nuclei, as disclosed inJP-A-1-214110 (the term "JP-A-" as used herein signifies "an unexaminedpublished Japanese patent application"). However, it was found thatsubstantially no crystal growth proceed at a temperature of 600° C. orlower. In the case of a silicon based material, in general, thecrystallization thereof proceeds in such a manner that the molecularchains in an amorphous state are once cut, and after establishing astate in which no recombination between the cut molecules occurs, themolecules are assembled into a part of a crystal in accordance with acrystalline molecule. The energy necessary to maintain the once cutmolecular chains to stand alone is quite large, and hence, this step ofmaintaining the cut molecules separated from each other is the barrierin the crystallization reaction. This energy corresponds to a heating ata temperature of about 1,000° C. for a duration of several minutes, orto a heating at about 600° C. for several tens of hours. Since theheating duration depends exponentially on the heating temperature, i.e.,energy, it was found practically unfeasible to perform thecrystallization at a temperature not higher than 600° C., morespecifically for example, at 500° C. A prior art concept of solid phaseepitaxial crystallization also failed to provide a solution to thisproblem.

The present inventors considered, apart from the conventionallyestablished theory of solid phase crystallization, of a means oflowering the barrier energy of the aforementioned process by a catalyticreaction. Thus, the present inventors noticed that nickel (Ni), iron(Fe), cobalt (Co), platinum (Pt) and palladium (Pd) have good affinitywith silicon and that they may easily form a silicide. In the case ofnickel, silicon readily forms nickel silicide (NiSi_(X) ; where0.4≦x≦2.5). Moreover, the present inventors noticed that the latticeconstant of nickel silicide is near to that of silicon. Accordingly, thefree energy of a ternary crystalline silicon-nickel silicide-amorphoussilicon system was simulated to confirm that amorphous silicon easilyundergoes reaction with nickel silicide at the phase boundary to formnickel silicide and crystalline silicon according to the chemicalreaction expressed by:

    amorphous silicon (silicon A)+nickel silicide (silicon B)→nickel silicide (silicon A)+crystalline silicon (silicon B)

wherein, silicon A and silicon B indicates the location of the siliconatoms. The potential barrier for this reaction is sufficiently low, andthe reaction takes place at a low temperature.

The reaction expressed by the formula above suggests that nickel atomsconvert amorphous silicon into crystalline silicon. In practice, thereaction is initiated at a temperature not higher than 580° C.; thereaction is observed to occur even at a temperature as low as 450° C.More typically, the crystallization can be effected at a temperaturelower than the crystallization temperature of a conventional amorphoussilicon by 20 to 150° C. As a matter of course, the reaction proceedsfaster with elevating the crystallization temperature. This is clearlyillustrated in FIG. 3 (which is to be referred in the Exampleshereinafter). Similar effects were obtained in cases using platinum(Pt), iron (Fe), cobalt (Co) or palladium (Pd).

The process according to the present invention is characterized in thatthe crystal growth proceeds isotropically to cover a circular area. Thisis because nickel atoms and the like move isotropically, and istherefore different from the conventional crystallization in which thecrystals grow along the lattice plane of the crystal.

In the process according to the present invention, nickel, iron, cobalt,platinum or palladium, either as pure metal or as a silicide thereof inthe form of a film, particles, clusters, etc., is provided in anisland-like, a stripe-like, a linear, or a dot-like morphology, so thatit may provide the starting point to develop and extend the area ofcrystallization into the peripheral portions thereof.

As described in the foregoing, the crystalline silicon thus obtained inthe process above differs from a conventional one obtained by solidphase epitaxial growth, however, it still has an excellent structuralcontinuity and a crystal similar to that of a single crystal silicon.Thus, the crystalline silicon obtained by the present process can besuitably used in the fabrication of a semiconductor device such as aTFT. Furthermore, it is found difficult to obtain a thin film ofimproved degree of crystallization (crystallinity) when the materialcontaining nickel, iron, cobalt, platinum or palladium is spreaduniformly over the substrate, because there are provided an infinitenumber of crystallization cites. The difference between the case ofproviding the material containing nickel, iron, cobalt, platinum orpalladium in a uniform thin film covering the entire surface and thecase of providing the material in an island-like, a stripe-like, linear,or a dot-like pattern, is clearly observed by means of Raman scatteringspectroscopy and X-ray diffraction analysis. It is confirmed by theseanalytical means that a superior crystalline silicon is obtained by theprocess according to the present invention.

The amorphous silicon film to be used as the starting material in thecrystallization process according to the present invention preferablycontains hydrogen at a concentration as low as possible. However, sincehydrogen is discharged from the amorphous silicon film with progressivecrystallization, no clear correlation was observed between the hydrogenconcentration of the starting amorphous silicon film and that of thesilicon film obtained by the crystallization. The hydrogen concentrationof the crystalline silicon film obtained by the process according to thepresent invention was typically in the range of from 1×10¹⁵ atoms·cm⁻³to 5% by atomic. A silicon film with still superior crystallinity can beobtained by reducing the concentration of carbon, oxygen, and nitrogen,each to a concentration of 1×10¹⁹ cm⁻³ or lower. Thus, the materialcontaining nickel, iron, cobalt, or platinum must be selected takingthis point into account.

However, the catalytic elements above, i.e., nickel, iron, cobalt,platinum and palladium themselves are not favorable for silicon.Accordingly, their concentration is preferably suppressed as low aspossible. The present inventors have found, through an extensive study,that the concentration of these elements for a semiconductor to be usedin a semiconductor device such as a TFT is preferably controlled, fromthe viewpoint of assuring favorable characteristics and reliability, tobe in the range of from 1×10¹⁵ atoms·cm⁻³ to 1 atom %, and morepreferably, in the range of from 1×10¹⁵ to 1×10¹⁹ atoms·cm⁻³ as observedby SIMS (secondary ion mass spectrometer). If the concentration of thecatalytic metal elements should fall below this range, sufficientcrystallization would not result. If the concentration should exceedthis range, on the other hand, semiconductors with poor characteristicsand reliability would be obtained.

Since nickel silicide at the final end of crystallization easilyundergoes dissolution in hydrofluoric acid or hydrochloric acid as aresult of the reaction expressed by the formula above, nickel can bereduced from the substrate by a treatment using these acids.

A film of a substance containing nickel, iron, cobalt, platinum orpalladium can be formed by any of the known physical and chemical means.For instance, usable is a method using a vacuum apparatus such as vaporphased deposition, sputtering, and CVD (chemical vapor deposition), or amethod which can be performed under the atmosphere such as spin-coating,dipping (coating), doctor blade process, screen printing, and spraypyrolysis.

In particular, spin-coating or dipping process can provide films havinga uniform thickness, and yet, the concentration of the resulting filmcan be precisely controlled. The solutions to be used in the abovemethods include such prepared by dissolving or dispersing an acetate, anitrate, or a carboxylate of nickel, iron, cobalt, platinum orpalladium, into a solvent such as water, an alcohol (either a lower or ahigher), and a petroleum solvent which may be a saturated hydrocarbon oran unsaturated hydrocarbon.

In using such substances, however, it has been feared that oxygen andcarbon included in the salt may diffuse into the silicon film and thatthey may impair the semiconductor characteristics of the silicon film.Accordingly, the present inventors conducted a study usingthermogravimetry and differential thermal analysis to find that, byproperly selecting the material, such additional substances decompose toyield oxides or elements at a temperature of 450° C. or lower, and thatno further diffusion of such substances occurs to allow them penetrateinto the silicon film. In particular, when decomposed under a reducingatmosphere such as nitrogen gas, salts such as acetates and nitrateswere found to yield elemental metal at a temperature of 400° C. orlower. These salts were found to yield oxides at first throughdecomposition in an oxygen atmosphere, but they finally yieldedelemental metal at higher temperatures upon the desorption of oxygen.

In employing the crystalline silicon film thus fabricated by the processaccording to the present invention in a semiconductor device such as aTFT, it can be seen that the final end of the crystallized portion isnot favorable to provide a semiconductor device. As described in theforegoing, this is because the crystallization fronts initiated from aplurality of starting points collide with each other at the final end ofthe crystallized portion, thereby providing a large grain boundary or adiscontinuity in crystallinity. Furthermore, the nickel concentration ofthis portion is high. Accordingly, the process for fabricating asemiconductor device according to the present invention requires topreviously optimize the pattern of the semiconductor device and that ofthe coating containing nickel, i.e., the starting points ofcrystallization.

The present invention is illustrated in greater detail referring tonon-limiting examples below. It should be understood, however, that thepresent invention is not to be construed as being limited thereto.

EXAMPLE 1

The present Example relates to a process for forming a plurality ofisland-like nickel films on a Corning #7059 glass substrate and thencrystallizing an amorphous silicon film using these films as thestarting points. The present Example also provides a process forfabricating a TFT using the thus obtained crystalline silicon film. Theisland-like nickel films can be formed by employing either of the twomethods; i.e., forming the island-like nickel films on the amorphoussilicon film, or under the amorphous silicon film. FIG. 2(A-1) shows themethod for providing the island-like nickel films under the amorphoussilicon film, and FIG. 2(A-2) shows the one for forming them on theamorphous silicon film. In the latter method, however, it should betaken into consideration that the etching of the thus formed nickel filmon the amorphous silicon is performed as a step subsequent to theformation of the nickel films. It follows that, though at a smallamount, unfavorable nickel silicide is formed through the reactionbetween nickel and amorphous silicon. Since a silicon film having asufficiently high crystallinity as to satisfy the object of the presentinvention cannot be obtained if nickel silicide remains on the siliconfilm, the residual nickel silicide must be completely removed usinghydrochloric acid, hydrofluoric acid, and the like. Thus, a thinneramorphous silicon film results as compared to the initially depositedfilm.

At any rate, nickel or nickel silicide can be patterned by either of thetwo conventionally known processes, i.e., an etch-off process whichcomprises patterning a photoresist by photolithography after forming anickel film, and then etching the portions of the nickel film whichremained uncovered by the resist, and a lift-off process which comprisespatterning a photoresist by photolithography before forming a nickelfilm thereon, and then peeling off the underlying photoresist toselectively form the nickel film.

No problem as described above can be found on the former methodreferring to FIG. 2(A-1). In a process according to this method, also,the nickel film other than the island-like portions is preferablyremoved completely. Furthermore, the substrate is treated using anoxygen plasma or ozone and the like to oxidize the region other than theisland-like regions and suppress the influence of the residual nickel.

At any rate, both processes comprise depositing a 2,000 Å thick siliconoxide base film 1B on a Corning #7059 substrate 1A by plasma CVD, and anamorphous silicon film 1 was further deposited thereon at a thickness offrom 200 to 3,000 Å, preferably, at a thickness of from 500 to 1,500 Å.The amorphous film could be more easily crystallized by removinghydrogen from the amorphous film by annealing the film at a temperatureof from 350 to 450° C. for a duration of from 0.1 to 2 hours.

In a process referring to FIG. 2(A-1), prior to the formation of theamorphous silicon film 1, a nickel film was deposited by sputtering at athickness of from 50 to 1,000 Å, preferably, from 100 to 500 Å. Theresulting nickel film was patterned to form an island-like nickel region2. The resulting structure as viewed from the upper side is shown inFIG. 1(A).

The island-like nickel portions are each formed in squares 2×2 μm² insize and taking a distance of from 5 to 50 μm, more specifically, forexample, 20 μm, between each other. A similar effect was obtained byusing nickel silicide in the place of nickel. Furthermore, favorableresults were obtained by heating the substrate in the temperature rangeof from 100 to 500° C., preferably in the range of from 180 to 250° C.This is ascribed to an increase in the tightness of adhesion between thebase silicon oxide film and the nickel film, and to the formation ofnickel silicide through the reaction between silicon oxide and nickel.Similar effects can be obtained by using silicon nitride, siliconcarbide, or silicon in the place of silicon oxide.

The resulting structure was annealed in the temperature range of from450 to 650° C., specifically, for example, at 550° C. for a duration of8 hours in a nitrogen atmosphere. The intermediate state during heatingis illustrated in FIG. 2(B). It can be seen in FIG. 2(B) that nickelproceeds to the central portion to form nickel silicide 3A from anisland-like nickel film which was located at the end of FIG. 2(A).Furthermore, it can be also seen that the portion 3 through which nickelhad passed provides crystalline silicon. Thus, the crystallization iscompleted at the point the advancing fronts initiated from two differingisland-like portions collide with each other and provide a residualnickel silicide 3A at the center. This is shown in FIG. 2(C).

FIG. 4 and FIG. 5 each provide the Raman scattering spectrogram and theX-ray diffractogram, respectively, of the resulting crystalline siliconfilm. In FIG. 4, the curve marked with C-Si corresponds to the Ramanspectrum of a standard sample, i.e., single crystal silicon. The curvesindicated by (a) and (b) each represent the Raman spectra for a siliconfilm obtained by the process according to the present invention, andthat for the non-crystallized regions. It can be seen clearly from theresults that the process according to the present invention provides afavorable silicon crystal.

FIG. 1(B) provides the structure obtained up to the present step, asviewed from the upper side of the substrate. In FIG. 2(C), nickelsilicide 3A in FIG. 2(C) corresponds to the grain boundary 4. By furthercontinuing the annealing, nickel moves along the grain boundary 4 andaggregates in the intermediate region 5 of the island-like nickelportions which are all deformed from their initial shapes.

A crystalline silicon can be obtained by the steps as described in theforegoing. It is not preferred, however, for the crystallin silicon tohave nickel diffused into the semiconductor coating from the thus formednickel silicide 3A. Accordingly, it is preferred that the etching issubjected to etching using hydrofluoric acid or hydrochloric acid,because these acids do not affect the silicon film. The structure thusobtained by etching is shown in FIG. 2(D). A groove 4A is obtained inthe place of the former grain boundary. It is not preferred to form thesemiconductor regions (active layer and the like) in such a manner thatthe groove is incorporated between the semiconductor regions. An examplefor the arrangement of the TFTs is shown in FIG. 1(C). The gateconnection 7, on the other hand, can cross the grain boundary 4.

An amorphous silicon film was crystallized according to the above meansusing a 2×2-μm² nickel region as the starting area. The dependence ofthe crystallization rate on the annealing temperature was studied. Thecrystallization rate was calculated by measuring the duration ofannealing necessary for the crystallization front to reach a distance offrom 10 to 50 μm away from the nickel region. The results are given inFIG. 3 as an example. Two types of amorphous silicon films, one having athickness of 500 Å and the other having a thickness of 1,500 Å, wereprepared to compare the results. As a matter of course, thecrystallization rate is greater for a higher annealing temperature. Thecrystallization rate also depends on the film thickness, and thecrystallization occurs more easily with increasing the film thickness.Since a practical semiconductor typically has a size of 50 μm cr less, acrystallization rate of at least 20 μm/hr is necessary if the annealingis performed for a duration of 5 hours. For a silicon having a thicknessof 1,500 Å, it can be read from FIG. 3 that the annealing must beperformed at a temperature of 550° C. or higher.

EXAMPLE 2

The present example relates to a process for fabricating a crystallinesilicon film using the constitution described in Example 1, except thatthe crystallinity of the film is further improved by irradiating a laserbeam after once crystallizing the film by heating. The steps andconditions of the steps other than that of the laser irradiation are thesame with those described in FIG. 1. The symbols and numbers in FIG. 6corresponds to the same referred to in Example 1.

Referring to FIG. 6, the process steps for fabricating a semiconductoraccording to the present example is described below. The steps (A-1)through (B) are the same as those explained in Example 1. Afterconducting the step with reference to FIG. 6(B), crystals were allowedto grow along the transverse direction, and a laser beam was irradiatedthereto to further ameliorate the crystallinity of the silicon film.Thus, a KrF excimer laser was operated to irradiate a laser beam at awavelength of 248 nm and at a pulse width of 20 nsec to the surface ofthe resulting crystalline silicon film to further accelerate thecrystallization thereof. The laser beam was irradiated at an outputenergy density of from 200 to 400 mJ/cm², for instance 250 mJ/cm² inthis case, for 2 shots. During the laser beam irradiation, the substratewas maintained at a temperature of from 150 to 400° C., morespecifically, for example, at 200° C., by heating, thereby fullyenhancing the effect of laser beam irradiation.

Usable laser light other than that of the KrF excimer laser aboveinclude those emitted from a XeCl excimer laser operating at awavelength of 308 nm and an ArF excimer laser operating at a wavelengthof 193 nm. Otherwise, an intense light may be irradiated in the place ofa laser light. In particular, the application of RTA (rapid thermalannealing) which comprises irradiating an infrared light is effectivebecause it can selectively heat the silicon film in a short period oftime.

Thus, a silicon film having a favorable crystallinity can be obtained byemploying any of the aforementioned methods. The previously crystallizedregion 3 obtained as a result of thermal annealing was found to changeinto a silicon film having a further improved crystallinity. On theother hand, the region (not shown in the figure) which did notcrystallize during the thermal annealing was found to yield apolycrystalline film as a result of laser irradiation. Raman scatteringspectroscopy revealed that the silicon film undergoes modification, butthat the crystallinity of the thus obtained polycrystalline film ispoor. Furthermore, observation by transmission electron microscoperevealed that a large number of fine crystals form in the film which wassubjected to laser irradiation without once crystallizing it by thermaltreatment. In contrast to this, relatively large grains of orientedcrystallites were found to constitute the thermally annealed andlaser-irradiated film 3 obtained according to the present invention.

After the completion of laser irradiation, the front end 3A of crystalgrowth was etched using hydrofluoric acid or hydrochloric acid. Thestructure obtained by etching is shown in FIG. 6(D).

Thus, a TFT was fabricated from a silicon film 3 shaped into anisland-like morphology. A remarkable increase in device characteristicswas observed on this TFT. More specifically, an N-channel TFT obtainedby employing the crystallization step described in Example 1 yields afield-effect mobility of from 50 to 90 cm² /Vs, and a threshold voltageof from 3 to 8 V. These values are in clear contrast to a mobility offrom 150 to 200 cm² /Vs and a threshold voltage of from 0.5 to 1.5 Vobtained for the N-channel TFT fabricated in accordance with the presentExample. The mobility is considerably increased, and the fluctuation inthe threshold voltage is greatly reduced.

Previously, the aforementioned TFT characteristics of such a high levelhad to be obtained from amorphous silicon film by laser crystallization.However, the silicon films obtained by a prior art laser crystallizationyielded fluctuation in the characteristics. Furthermore, thecrystallization process required an irradiation of a laser light at anenergy density of 350 mJ/cm² or higher at a temperature of 400° C. orhigher, and it was therefore not applicable to mass production. Incontrast to the conventional processes, the process for fabricating aTFT according to the present Example can be performed at a substratetemperature and at an energy density both lower than the correspondingvalues of the conventional processes. Accordingly, the process accordingto the present invention is suitable for mass production. Furthermore,the quality of the devices obtained by the present process is as uniformas the one for the devices obtained by a conventional solid phase growthcrystallization using thermal annealing. Accordingly, TFTs of uniformquality can be obtained stably.

In the present invention, the crystallization was found to occurinsufficiently when the nickel concentration was low. However, theprocess according to the present Example employs laser irradiation tocompensate for an insufficient crystallization. Accordingly, TFTs withsatisfactorily high quality can be obtained even when the nickelconcentration is low. This signifies that devices containing nickel at astill lower concentration can be implemented, and that devices havingexcellent electric stability and reliability can be obtained.

EXAMPLE 3

The present example relates to a process for introducing a catalyticelement into the amorphous film by coating the upper surface of theamorphous silicon film with a solution containing a catalytic elementwhich accelerates the crystallization of the amorphous silicon film.Nickel is used as the catalytic element in this Example. The presentExample is mostly the same as that described in Example 1, except forthe method of introducing nickel. The crystallization step and the stepssubsequent thereto are the same as those described in Example 1. It isalso the same as in Example 1 that the structure viewed from the upperside of the substrate corresponds to FIG. 1.

FIG. 7 shows schematically the step-sequential fabrication processaccording to the present invention. A silicon oxide film 1B as a basecoating was deposited on a 10×10-cm² square Corning #7059 glasssubstrate 1A, and a 1,000 Å thick amorphous silicon film 1 was depositedfurther thereon by plasma CVD.

A silicon oxide film 13 was deposited on the resulting amorphous siliconfilm to a thickness of 1,200 Å to provide a mask. A silicon oxide film13 as thin as 500 Å in thickness can be used without any problem, andthe film can be made even thinner by using a denser film.

The resulting silicon oxide film 13 was patterned as desired by anordinary photolithographic patterning. Then, a thin silicon oxide film12 was deposited in an oxygen atmosphere by irradiating ultraviolet (UV)light. More specifically, the silicon oxide film 13 was fabricated byirradiating the UV light for 5 minutes. The silicon oxide film 12 isbelieved to have a thickness of from 20 to 50 Å.

The silicon oxide film above is formed for improving wettability of thepattern with the solution to be applied hereinafter. Accordingly, a 5-mlportion of an acetate solution 11 containing 100 ppm by weight of nickelwas dropped on the surface of a 10×10-cm² square substrate. A spinner 10was operated for 10 seconds at a rate of 50 rpm to form a uniformaqueous film on the entire surface of the substrate. The spinner 10 wasoperated for an additional 60 seconds at a rate of 2,000 rpm to effectspin drying after maintaining the substrate for 5 minutes. The substratemay be subjected to rotation at a rate of from 0 to 150 rpm on aspinner. This step is illustrated in FIG. 7(A).

Subsequent to the removal of the silicon oxide mask 13, the resultingstructure was subjected to heat treatment at 550° C. under a nitrogenatmosphere for a duration of 4 hours to crystallize the amorphoussilicon film 1. In this manner, crystallization is allowed to occur fromthe region 14 into which nickel was introduced to the region into whichnickel was not introduced along the transverse direction.

Referring to FIG. 7(B), it can be seen that the crystallization isinitiated from the region 14 into which nickel was directly introduced,and that it proceeds transversely towards the central portion. Acrystalline silicon film 3 is obtained in this manner. Silicon nitrideis formed in the region 3A at which the growth fronts of the crystalscollide with each other.

Then, the region 3A of nickel nitride was removed using hydrofluoricacid or hydrochloric acid. The structure thus obtained by etching isshown in FIG. 7(D).

The concentration profile of nickel in this region is shown in FIG. 8.The nickel concentration in the crystalline silicon film upon completionof the crystallization was examined by SIMS (secondary ion massspectroscopy). It is confirmed that the nickel concentration of theregion 14 into which nickel was directly introduced yield a value stillhigher than the concentration shown in FIG. 8 by a digit or more.

It is also effective to further improve the crystallinity of thecrystalline silicon film obtained above by irradiating a laser beam oran intense light equivalent thereto in the same manner as in theforegoing Example 2. In the case of Example 2, the morphology of thefilm was impaired by laser irradiation because grains of nickel silicidefrom about 0.1 to 10 μm precipitated from the nickel film having arelatively high nickel concentration. However, since the nickelconcentration of the nickel film can be far reduced as compared withthose obtained in Examples 1 and 2, the precipitation of nickel silicideand hence surface roughening can be prevented from occurring.

The nickel concentration as shown in FIG. 8 can be controlled bychanging the nickel concentration of the solution to be applied. In thepresent invention, the nickel concentration in the solution was adjustedto 100 ppm. However, it is confirmed that the crystallization alsooccurs even when the concentration is decreased to 10 ppm.Crystallization occurs in the same manner by using a solution containingnickel at a concentration of 10 ppm. In this case, the nickelconcentration as shown in FIG. 8 can be further lowered by a digit.However, the use of a solution containing nickel at too low aconcentration shortens the distance of crystal growth along thetransverse direction indicated by the arrow, and is therefore notdesired.

An acetate solution was used in the present example as a solutioncontaining the catalytic element. However, other usable solutionsinclude an aqueous solution selected from a wide variety, and a solutioncontaining an organic solvent. The catalytic element need not necessarybe included as a compound, and it may be simply dispersed in thesolution.

The solvent for the catalytic element can be selected from the groupconsisting of polar solvents, i.e., water, alcohol, acids, and ammoniawater.

When nickel is used as the catalytic element, nickel is incorporatedinto a polar solvent in the form of a nickel compound. The nickelcompound is selected from, representatively, the group consisting ofnickel bromide, nickel acetate, nickel oxalate, nickel carbonate, nickelchloride, nickel iodide, nickel nitrate, nickel sulfate, nickel formate,nickel acetylacetonate, nickel 4-cyclohexylbutyrate, nickel oxide, andnickel hydroxide.

The solvent may be selected from a non-polar solvent selected from thegroup consisting of benzene, toluene, xylene, carbon tetrachloride,chloroform, and ether.

In this case, nickel is involved in the solution in the form of a nickelcompound, which is selected from the group consisting of nickelacetylacetonate, and nickel 2-ethylhexanate.

It is also effective to add a surfactant into the solution containing acatalytic element. A surfactant increases the adhesiveness of thesolution to the surface of the silicon oxide film, and controls theadsorptivity. The surfactant may be applied previously to the surface tobe coated. If elemental nickel is used as the catalytic element, it mustbe previously dissolved into an acid to obtain a solution thereof.

Instead of using a solution containing nickel completely dissolved intothe solution, an emulsion, i.e., a material comprising a dispersingmedium uniformly dispersed therein a powder of metallic nickel or of anickel compound, can be used.

The same applies in other cases using a material other than nickel asthe catalytic element.

A solution containing a non-polar solvent, i.e., a toluene solution ofnickel 2-ethylhexanate, can be directly applied to the surface of anamorphous silicon film. In this case, it is effective to use a materialsuch as an adhesive generally employed in forming a resist. However, theuse of the adhesive in an excess amount reversely interferes thetransfer of the catalytic element into amorphous silicon.

The catalytic element is incorporated into the solution approximately inan amount as nickel of, though depending on the type of the solution,from 1 to 200 ppm by weight, and preferably, from 1 to 50 ppm by weight.This range of addition is determined by taking the nickel concentrationof the crystallized film and the resistance against hydrofluoric acidinto consideration.

As described in the foregoing, the present invention is epoch-making inthat it enables the crystallization of an amorphous silicon to takeplace at an even lower temperature and within a shorter period of time.Furthermore, the process according to the present invention is suitablefor mass production, and yet, it can be performed employing the mostcommonly used equipments, apparatuses, and methods. Accordingly, it is apromising and a beneficial process for the electronic industry.

More specifically, for instance, a conventional solid phase growthprocess requires an annealing step for a duration of at least 24 hours.Considering that the process time per substrate is preferably 2 minutes,15 annealing furnaces are necessary to make the process practicallyfeasible. However, the present invention allows the process to completewithin 8 hours, and under optimal conditions, the process can be evenmore shortened to a mere 4 hours or less. This signifies that theprocess can be performed while reducing the number of furnaces to only asixth or less of the above calculated number. This leads to an increaseof productivity and the cutting down of equipment investment, therebylowering the process cost of the substrates. Accordingly, economicalTFTs can be produced, and this might call novel demands. Conclusively,the present invention is greatly beneficial for the industry.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed:
 1. A method for fabricating a semiconductor comprisingthe steps of:forming a semiconductor film comprising silicon on aninsulating surface; disposing a material for promoting crystallizationof silicon in contact with said semiconductor film; annealing saidsemiconductor film to crystallize said semiconductor film with saidmaterial; and removing said material from the crystallized semiconductorfilm using an acid solution, wherein said material is selected from thegroup consisting of nickel, iron, cobalt, platinum and palladium.
 2. Amethod according to claim 1 wherein said acid solution is selected fromthe group consisting of hydrofluoric acid and hydrochloric acid.
 3. Amethod according to claim 1 wherein said annealing step is carried outat a temperature of 450 to 650° C.
 4. A method according to claim 1wherein the material is disposed in contact with said semiconductor filmin a form selected from the group consisting of a film, particles, andclusters.
 5. A method according to claim 1 wherein said material isdisposed in contact with the semiconductor film in a pattern selectedfrom the group consisting of an island-like, a stripe-like, a linear,and a dot-like pattern.
 6. A method according to claim 1 wherein saidinsulating surface is a silicon oxide film formed over a substrate.
 7. Amethod according to claim 1 wherein said semiconductor film is in anamorphous state.
 8. A method according to claim 1 wherein saidsemiconductor film contains carbon, oxygen, and nitrogen at aconcentration of 1×10¹⁹ cm⁻³ or lower, respectively.
 9. A method forfabricating a semiconductor device comprising:forming a semiconductorfilm comprising silicon on an insulating surface; disposing a materialfor promoting crystallization of silicon in contact with saidsemiconductor film; annealing said semiconductor film to crystallizesaid semiconductor film; removing said material from the crystallizedsemiconductor film using an acid solution; and patterning saidcrystallized film to form active layers of the semiconductor device,wherein said material is selected from the group consisting of nickel,iron, cobalt, platinum and palladium.
 10. A method according to claim 9wherein said acid solution is selected from the group consisting ofhydrofluoric acid and hydrochloric acid.
 11. A method according to claim9 wherein said annealing step is carried out at a temperature of 450 to650° C.
 12. A method according to claim 9 wherein the material isdisposed in contact with said semiconductor film in a form selected fromthe group consisting of a film, particles, and clusters.
 13. A methodaccording to claim 9 wherein said material is disposed in contact withthe semiconductor film in a pattern selected from the group consistingof an island-like, a stripe-like, a linear, and a dot-like pattern. 14.A method according to claim 9 wherein said insulating surface is asilicon oxide film formed over a substrate.
 15. A method according toclaim 9 wherein said semiconductor film is in an amorphous state.
 16. Amethod according to claim 9 wherein said semiconductor film containscarbon, oxygen, and nitrogen at a concentration of 1×10¹⁹ cm⁻³ or lower,respectively.
 17. A method for fabricating a semiconductorcomprising:forming a semiconductor film comprising silicon on aninsulating surface; preparing a solution by dissolving a material forcrystallization of silicon in a solvent; applying said solution incontact with said semiconductor film; annealing said semiconductor filmto crystallize said semiconductor film wherein crystallization ofsilicon proceeds with diffusion of said material through saidsemiconductor film; and removing said material from said semiconductorfilm using an acid solution, wherein said material is selected from thegroup consisting of nickel, iron, cobalt, platinum and palladium.
 18. Amethod according to claim 17 wherein said solution further contains asurface active agent.
 19. A method according to claim 17 wherein saidacid solution is selected from the group consisting of hydrofluoric acidand hydrochloric acid.
 20. A method according to claim 17 wherein saidannealing step is carried out at a temperature of 450 to 650° C.
 21. Amethod according to claim 17 wherein said insulating surface is asilicon oxide film formed over a substrate.
 22. A method according toclaim 17 wherein said semiconductor film is in an amorphous state.
 23. Amethod according to claim 17 wherein said semiconductor film containscarbon, oxygen, and nitrogen at a concentration of 1×10¹⁹ cm³ or lower,respectively.
 24. A method of manufacturing a semiconductor devicecomprising the steps of:forming a semiconductor film comprising siliconon an insulating surface, said semiconductor film including a region tobecome an active region of said semiconductor device; disposing amaterial for promoting crystallization of silicon in contact with saidsemiconductor film; heating said semiconductor film to crystallize thesemiconductor film wherein crystallization of silicon proceeds with thediffusion of said material through said semiconductor film; removing atleast a portion of the crystallized semiconductor film by using an acidsolution; and patterning said crystallized semiconductor film to formactive layers of the semiconductor device, wherein said material isselected from the group consisting of nickel, iron, cobalt, platinum andpalladium.
 25. A method according to claim 24 wherein said acid solutionis selected from the group consisting of hydrofluoric acid andhydrochloric acid.
 26. A method according to claim 24 wherein saidannealing step is carried out at a temperature of 450 to 650° C.
 27. Amethod according to claim 24 wherein the material is disposed in contactwith said semiconductor film in a form selected from the groupconsisting of a film, particles, and clusters.
 28. A method according toclaim 24 wherein said material is disposed in contact with thesemiconductor film in a pattern selected from the group consisting of anisland-like, a stripe-like, a linear, and a dot-like pattern.
 29. Amethod according to claim 24 wherein said insulating surface is asilicon oxide film formed over a substrate.
 30. A method according toclaim 24 wherein said semiconductor film is in an amorphous state.
 31. Amethod according to claim 24 wherein said semiconductor film containscarbon, oxygen, and nitrogen at a concentration of 1×10¹⁹ cm⁻³ or lower,respectively.